When a light beam generated by a coherent light source passes through an optical system, it acquires a grain-like structure (so-called speckle structure) with 100% modulation of the light intensity, as well as nonuniformities associated with such phenomena as light interference. This phenomenon prevents such light from direct use, e.g., in image-forming systems. The aforementioned speckle structure is caused by diffraction of a coherent wavefront in the areas of external and internal (structural) heterogenuieties of optical system elements with subsequent interference of diffraction beams.
Coherence is correlation between the phases of two or more waves or between the phases of parts of a single wave so that interference effect may be produced between them.
Since the aforementioned micro- and macro-heterogenuieties cannot be completely eliminated, the only countermeasure that can be undertaken for the solution of the above problem is to reduce the depth of radiation modulation and the frequency of interference fringes by decreasing spatial coherence in the initial light beam. At the same time, it would be desirable to ensure uniform distribution of brightness in the cross section of the light beam in an object or illumination plane.
Many attempts have been made to solve the problem associated with the speckle structure. For example, U.S. Pat. No. 4,155,630 issued in 1979 to Charles Ih discloses a process and apparatus for improving image creation in a coherent light imagery system which involves directing a diffused light onto a mirror having a rocking motion that will cause the reflected rays to sweep a two-dimensional area and focusing the reflected light through a diffuser before collimating same for use in image creation. More particularly, this invention creates the rocking motion by applying a combination of voltages to three independent piezo-electric crystals upon which the mirror is mounted.
A disadvantage of the apparatus of U.S. Pat. No. 4,155,630 consists in that it utilizes piezo-elements which are expensive to manufacture, difficult to control and produce a limited shift insufficient to homogenize the light distribution.
U.S. Pat. No. 4,427,261 issued to Giok Khoe, et al. in 1984 describes an optical transmission system having reduced modal noise. In this device, an optical fiber is coupled to an end face of a semiconductor laser diode and the other end face is coupled to one end of a monomode optical waveguide. The other end of waveguide is terminated in a reflecting manner, e.g. by mirror, to reduce the spatial coherence length of the laser. According to the inventors, the speckle patterns are only produced in the transmission fiber over a short distance from the laser, and modal noise further on in the transmission fiber can be prevented by avoiding the use of non-ideal fiber couplings or other connections having a spatial filter effect within this short distance from the laser.
A disadvantage of the system is that, due to physical limitations inherent in mono-mode optical fiber, it is applicable only to semiconductor lasers and is unsuitable for more powerful wide-beam power sources, such as an excimer laser.
U.S. Pat. No. 4,511,220 issued in 1985 to Charles Scully discloses a laser target speckle eliminator with regard to laser light reflected from a distant target whose roughness exceeds the wavelength of the laser light. The apparatus includes a half plate wave member, a first polarizing beam splitter member, a totally reflecting right angle prism, and a second polarizing beam splitter member, all of which are in serial optical alignment, that are used in combination to convert a linearly (i.e., vertically) polarized light beam, which is emitted by a laser having a known coherence length, into two coincident, orthogonally polarized, beams that are not coherent with each other, and that have an optical path difference which exceeds the known coherence length of the emitting laser, to eliminate the speckle.
The main disadvantage of the apparatus of U.S. Pat. No. 4,511,220 is that splitting of the coherent light beam only into two components is insufficient for highly coherent lasers. Furthermore, this system does not improve uniformity at all.
U.S. Pat. No. 4,256,363 issued in 1981 to Robert Briones describes a speckle suppression of holographic microscopy in an apparatus of reconstructing and viewing a speckled holographic image through a microscope, with the result that the speckle of the holographic image is significantly reduced, without loss of resolution of the image. A finely-structured and transparent light diffuser is interposed through the aerial image formed by a hologram or an image formed or relayed by a lens system such as a microscope objective prior to the eyepiece. This diffuser is moved in its plane with a rotary or vibratory motion to suppress the speckle.
The principle described in U.S. Pat. No. 4,256,363 is applicable only to microscopic optical system with an extremely small field of view. In the case of large fields of view this system will not provide uniformity in the light distribution pattern. Another disadvantage of the aforementioned apparatus is that it utilizes a single-stage coherence reducing unit which is insufficient for suppressing coherence to a required degree.